Field
The technique disclosed herein relates to an image generation method, an image generation apparatus, and a storage medium.
Description of the Related Art
Optical coherence tomography (OCT) has been put to practical use as a method for acquiring a tomogram of a measurement target such as a living body in a non-destructive and non-invasive manner. OCT is widely used particularly in the field of ophthalmology, for example, in an opthalmological diagnosis of a retina, in which a tomogram of the retina at an ocular fundus of a subject's eye is acquired.
In OCT, light reflected by a measurement target and light reflected by a reference mirror are caused to interfere with each other, an interference signal is detected, and time dependency or wavenumber dependency of the intensity of interfered light is analyzed to acquire a tomogram. An example of an optical coherence tomogram acquiring apparatus is a time domain OCT (TD-OCT) apparatus that acquires depth information about a measurement target by changing the position of a reference mirror. Another example is a spectral domain OCT (SD-OCT) apparatus that uses a broadband light source. Still another example is a swept source OCT (SS-OCT) apparatus that uses, as a light source, a wavelength-variable light source device capable of changing an oscillation wavelength. SD-OCT and SS-OCT are collectively referred to as Fourier domain OCT (FD-OCT).
In recent years, pseudo angiography using FD-OCT has been suggested, which is called OCT angiography (OCTA).
Fluoroangiography, which is angiography typically employed in modern clinical medicine, is performed by injecting a fluorescent dye (for example, fluorescein or indocyanine green) into a body or a part thereof. In fluoroangiography, blood vessels through which the fluorescent dye flows are two-dimensionally displayed. In contrast, OCT angiography enables pseudo angiography in a non-invasive manner and also enables three-dimensional display of a blood-flow network. Furthermore, OCT angiography produces a higher resolution result than fluoroangiography and is able to depict micro-vessels or blood flows at an ocular fundus.
As for OCT angiography, a plurality of methods have been suggested in accordance with a difference in a blood flow detection method. Fingler et al. “Mobility and transverse flow visualization using phase variance contrast with spectral domain optical coherence tomography”, Optics Express, Vol. 15, No. 20, pp. 12637-12653 (2007) discloses a method for separating an OCT signal from a blood flow by extracting only a signal component in which time modulation occurs from the OCT signal. Optics Letters Vol. 33, Iss. 13, pp. 1530-1532 (2008) “Speckle variance detection of microvasculature using swept-source optical coherence tomography” discloses a method using variations in phase due to blood flows. Further, Mariampillai et. al., “Optimized speckle variance OCT imaging of microvasculature”, Optics Letters 35, pp. 1257-1259 (2010) and United States Patent Application Publication No. 2014/221827 discloses a method using variations in intensity due to blood flows. In this specification, an image representing a signal component indicating time modulation in an OCT signal may be referred to as a motion contrast image, a pixel value of the motion contrast image may be referred to as a motion contrast value, and a data set of such motion contrast values may be referred to as motion contrast data.
In the above-described OCT angiography, however, it is difficult to easily acquire a motion contrast image suitable for diagnosis because, for example, a blood flow portion in the ocular fundus is not sufficiently clearly portrayed due to strong reflected light from a retinal pigment epithelium (RPE) or various noises.